GM-GMC: 1999 Chevy S-10, gasket leaks, coolant level

Question
My 1999 S-10 does not continually blow hot air when the heater is on.  It will often only blow cold air and then the engine temp will rise to an uncomfortable level (almost red line), but then the heat will kick in again and lower the engine level to a normal range.  What do you think is going on and what will it take to fix it.

Answer
Check your coolant level.  When the coolant level falls below a certain level the heater core is bypassed.  It would also explain your truck overheating if the coolant level was low.  If your coolant is that low, check to see where the coolant is going.  External leaks are not too bad, but those engines (4.3 and 2.2) were prone to internal gasket leaks also.  Check your oil, it shouldn't be milky white.
hope this helps.